netfilter: ipvs: Add Maglev hashing scheduler

Implements the Google's Maglev hashing algorithm as a IPVS scheduler.

Basically it provides consistent hashing but offers some special
features about disruption and load balancing.

 1) minimal disruption: when the set of destinations changes,
    a connection will likely be sent to the same destination
    as it was before.

 2) load balancing: each destination will receive an almost
    equal number of connections.

Seel also for detail: [3.4 Consistent Hasing] in
https://www.usenix.org/system/files/conference/nsdi16/nsdi16-paper-eisenbud.pdfSigned-off-by: Inju Song <inju.song@navercorp.com>
Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>